1. Field of the Invention
The present invention relates to the field of screw fasteners, particularly to fasteners for attaching wood substrates to metal substrates.
2. Description of the Related Art
Steel fasteners are known in the art for fastening wooden substrates, such as plywood or particleboard, to metal substrates, such as steel framing members. Commonly, these fasteners have an elongate shank and are designed to be axially driven by powered tools.
In many construction applications the metal support studs have been relatively thick, 12 gauge steel or thicker. A fastener for this application usually has an elongate shank having at least one threaded portion starting at an axial position near the head and axially extending along the shank, and many have included a set of wings that radially extend away from the drilling tip farther than the diameter of the threads so that the threads will not engage in the wood, which has been known to create difficulties drawing the wood towards the steel support. Most of the fasteners include a recess in the head to receive a bit from a fastener driving tool.
Examples of fasteners intended for this application include Illinois Tool Works Inc. part number 1082000, a 1 7/16 inch long fastener having wings for attaching plywood substrates up to 0.75 inches thick to steel, and Illinois Tool Works Inc. part numbers 1092000 and 1094000, a 2.25 inch long fastener and a 2.75 inch long fastener, respectively, each having wings, where both fasteners are for attaching thick wood substrates such as two-by-fours to thick steel. The wings are intended to bore out a hole with a diameter at least as large as the diameter of the threads so that they do not engage in the wood. After the wings hit the steel, they are intended to break off so they do not bore a hole in the steel larger than the threads, so that the threads will be able to tap the steel and clamp the steel and wood together. For each of these fasteners, the steel must be a certain thickness so that the wings will break off, at least 16 gauge steel for the 1 7/16 inch fastener, and at least 0.125 inches thick for the 2.25 and 2.75 inch fasteners.
Another fastener intended for attaching wood to steel support members is Hilti screw item number 00010429, a 3.5 inch screw having a 1.875 inch threaded portion proximate to the head of the fastener and a 1.5 inch long unthreaded portion from the threaded portion to the tip. The unthreaded portion of the Hilti screw is long enough to drill through the steel before the threads engage the wood. After pulling through the wood, the threads tap the steel and attach the wood to the steel.
Longer screws have had problems compared to shorter screws. First, longer screws are more expensive to manufacture because they required more material. Second, longer screws are harder to handle and keep stable as they are being driven into a substrate because they tend to wobble and not drive true. Third, longer screws have a longer drill time so that it is harder for an installer to quickly drive longer screws, making a job less efficient.
It has become a recent trend in the construction industry to use lighter steel, such as 18-22 gauge, for metal support members because lighter steel tends to be less expensive while still providing strength comparable to heavier steel. Winged fasteners, such as those described above, are not appropriate for attaching wood substrates to thinner steel because the wings on the drilling tip do not break off. The wings drill a hole in the metal that is larger than the threading, so that the threads cannot provide attachment to the steel. If the wings are removed, the long threaded portion described above tends to engage in the wood and continue to pull the fastener through the wood, or stop rotation of the fastener, causing the recess in the head to be stripped by the fastener driving tool. Also, the elongate threaded portion may not act to draw the metal support stud back towards the wood because the threads engage the wood and the steel, which tends to keep the wood and metal separated. Also, many applications require the attachment of a thick wood substrate, such as a two-by-four having a thickness of about 1.5 inches, to the thinner metal substrates described above.
What is needed is a self-drilling fastener for attaching wood with a thickness of about 1.5 inches, such as a two-by-four, to lighter gauge metal substrates that will draw the wood and the metal substrates towards each other and clamp the metal and wood together.